Glen-L.com

Rudder offset. I think I may have mis interpeted the Glen L book on inboard motor installations. The illustation indicates a rudder off set for a rigtht hand prop to be to the right, the caption says it should be oppisite. I have a left hand prop, and have offset the rudder to the left 1/4 inch. Now, I am second guessing, which is correct. I have a beam of 77 inches at the stern, will this 1/4 inch matter? I haven't drilled the bolt holes yet, so I could rework if I have to. please comment, thanks john bell

Look in "Shop Talk" in Webletter 95. It talks about rudder offset...offset from the centerline rather than angled...and references an article in Glen-L's book Inboard Motor Installations. You may want to hook up with Kens also, as I know his is offset on his Double Eagle.

Re-reading it, the article says they offset their rudder 1" from the C/L on a speed boat...I'm not sure how fars Kens did his. Looks like a good guess is in order...

Thanks for the replies. The illustration in my Book (1978 Glen L Inboard motor installation) Does not show a tilt, it shows a straight vertical, they have in the illustration a counterclockwise prop with the rudder offset to the right, they do not mention how much. They definately show vertical lines of the centerline of the boat and the rudder centerline, which are different, no tilt. My prop turns clockwise and I offset from the centerline of the boat 1/4 inch to the left. I lined the rudder hole with a piece of pvc kitchen sink pipe and it is very tight, no chance I could tilt as it is. Unless I cut the pipe hole liner out and go from there. I lined the hole to keep the wood dry. the pipe is embedded in epox. Course after re reading the caption, which says to offset to the OPPOSITE side of prop rotation, to counteract torque. Damn I'm confused agin. I will read the web letter and maybe get it straight in my mind. thanks, john bell

Read the webletter. the illustration there is the same one I have in my book. I think I may quit worrying about this. I don't have the prop shaft installed yet, the hole are all there, but I just epoxied the wood block for the shaft log in Saturday and it needs cleaning up a bit to try fit the shaft back in. Too damn cold in the shop to be fooling around out there right now. 19 this morning! I just know Minneapolis is colder but heck this old cowboy ain't used to it. Reaming the epox globs out did not seem the thing to do today, so I just polished brass in my easy chair!

I never really understood rudder offset. On my outboard there is a little rudder located behind the prop. It needs to be set slightly off straight to counter act the torque of the prop. This allows the boat to go straight when the engine is straight. If this was not adjusted correctly then the boat would not travel straight unless you held the wheel so that the engine was a bit off straight. Outboard engines want to be straight.

With my inboard I didn't worry about offsetting my rudder. To make my boat go straight I simply hold the steering wheel so that it goes staight, just like a car. If the rudder under the boat is ever-so-slightly turned to counter the torque effect then so be it. Since the prop is not attached to the rudder as it is in an outboard there is very little desire for the rudder to be pulled straight.

Bruce.

For what it's worth, this is from the Century Boat Club's "Common mistakes when restoring Century Boats".
Propeller Lateral Offset: In common terms it is how to get correct steering with inconsistent propeller thrust. A propeller does not give straight line thrust. There is more thrust on the downward side than the upward side. As a result of this the rudder must be off center of the prop or the steering characteristics can be altered. What Century did to achieve this was to put the engine and shaft in at a slight angle from straight. When you look at the rudder and shaft on a Century inboard the propeller should be off to the left of the rudder center. This amount was between 3/4" and 1". If this is not done the boat will steer much better to left than the right. Many restorers have aligned the engine in a straight line with the boat and rudder, this is a big mistake. Also another big problem is engine replacement. Many new engines are left hand rotation but the boats are set up for right-hand rotation. The problem is doubled. Steering becomes very bad to the left and very strong to the right. To correct this with left hand rotation engines you have to shift the engine the other way, re-locate the strut so the prop is off-set to the right
Another ways I've found to solve the problem are to use adjustable trim tabs and before I got them I angled (tilted) the rudder shaft by bending it slightly.
Bob Smith
Chester, Sc
Merry Christmas!
Most people are willing to take the Sermon on the Mount as a flag to sail under, but few will use it as a rudder by which to steer.
Oliver Wendell Holmes

The offset rudder helps you attain a neutral helm. You do not tilt the rudder. You offset it from centerline of shaft.
Offset the rudder into the upper prop blade.
Hence, I have a R/H prop and offset the rudder to the left. I drilled the rudder port 1 shaft dia. to the left. This allows me to pull the shaft past the rudder.
Another way to say it would be , the rudder side is tangent to the shaft.
BTW, my shaft is setup on centerline.

In other words, If I have a left hand rotating prop, and I have offset my rudder to the left, I done her wrong? Durn, that's what I thought. Rework here I come! thanks a lot. Better now than when the boat won't turn one way. Maybe I can up the horsepower and make a circle racer outa' her. john

I wouldn't get too worked up about it.... Have you ever driven a master craft or comparable competition ski boat. The first time I drove mine I thought it was a reject.... It pulls hard to one side and you cannot take your hands off the wheel or the boat will turn HARD. The rudder is off center by at least an inch. The reason.... So you can track perfectly straight through a slalom course. If the boat was true, you would constantly be turning from the left and right contact points to track straight down the channel. It's much easier to track straight when you only have to hold one edge.

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BUNMAN

My Tahoe has a LH prop with rudder offset about an inch to the right. Tracks pretty well. I just got lucky.

My thought at the time was about having room for a prop puller and getting a damaged shaft out without removing the rudder. To pull my rudder you must remove the batteries, the rear seat and frame, and drain and remove the fuel tank.

Bill

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Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

Bill:
I have R/H prop with rudder offset to the left. I had talked to Glen-L about just 'how much' offset. Glen-L had mentioned offsetting the hot boats approx 1".
I offset mine 1 1/4" for the 1.25 shaft.
Are we perhaps coming to a consensus the amount of offset = 1 shaft dia.?

The Century "solution" that I quoted above was to offset(angle) the line of the engine/shaft/prop from the boat centerline. The angle apparently produced about a 1" offset at the prop. I think some single engine aircraft use this to counter torque and "P" factor. Seems like some creativity in the strut mounting (the ability to vary the angle of the shaft relative to C/L) could offer post installation adjustment opportunities. Obviously the motor mounts would have to accommodate this.
Bob Smith
Chester, SC

A figure with curves always offers a lot of interesting angles.
Wesley Ruggles

I guess I cranked up a pretty good subject. I ended up with a perfectly aligned prop and rudder. My rudder is easily removed, nothing in the way, just two 5/16 bolts, and it drops out the bottom, so no really shaft or prop problems that I see. I am gonna try it and see what happens, if she pulls well, I guess I'll deal with that at the time. I have tried to load weight to the other side of the boat to compensate but the first run will tell. Got the engine running, with the shafts all aligned, so I will start on the topside wood next. One thing was I hooked up some 3 inch pvc as exhaust for temporary, this worked very good, didnt' get much over warm to the touch, makes me wonder if it would work permanant. Sure is cheaper than 3 inch hardwall. Anybody got any suggestions? john bell